// Physical memory allocator, for user processes,
// kernel stacks, page-table pages,
// and pipe buffers. Allocates whole 4096-byte pages.

#include "types.h"
#include "param.h"
#include "memlayout.h"
#include "spinlock.h"
#include "riscv.h"
#include "defs.h"

void freerange(void *pa_start, void *pa_end);

extern char end[];		// first address after kernel.
		   // defined by kernel.ld.

struct run {
	struct run *next;
};

struct {
	struct spinlock lock;
	struct run *freelist;
} kmem;

void kinit()
{
	initlock(&kmem.lock, "kmem");
	freerange(end, (void *)PHYSTOP);
}

void freerange(void *pa_start, void *pa_end)
{
	char *p;
	p = (char *)PGROUNDUP((uint64) pa_start);
	for (; p + PGSIZE <= (char *)pa_end; p += PGSIZE)
		kfree(p);
}

// Free the page of physical memory pointed at by v,
// which normally should have been returned by a
// call to kalloc().  (The exception is when
// initializing the allocator; see kinit above.)
void kfree(void *pa)
{
	struct run *r;

	if (((uint64) pa % PGSIZE) != 0 || (char *)pa < end
	    || (uint64) pa >= PHYSTOP)
		panic("kfree");

	// Fill with junk to catch dangling refs.
	memset(pa, 1, PGSIZE);

	r = (struct run *)pa;

	acquire(&kmem.lock);
	r->next = kmem.freelist;
	kmem.freelist = r;
	release(&kmem.lock);
}

// Allocate one 4096-byte page of physical memory.
// Returns a pointer that the kernel can use.
// Returns 0 if the memory cannot be allocated.
void *kalloc(void)
{
	struct run *r;

	acquire(&kmem.lock);
	r = kmem.freelist;
	if (r)
		kmem.freelist = r->next;
	release(&kmem.lock);

	if (r)
		memset((char *)r, 5, PGSIZE);	// fill with junk
	return (void *)r;
}
